1. Field of the Invention
The present invention relates to a method for depositing a tungsten suicide layer, and more particularly to a method for depositing a tungsten silicide layer on a polysilicon layer using dichlorosilane gas as a silane source gas so as to prevent void generation on an interfacial surface formed between the polysilicon layer and the tungsten silicide layer.
2. Description of the Related Art
Generally, in a semiconductor memory device such as a DRAM (dynamic random access memory), a multi-layer structure consisting of a polysilicon layer and a tungsten silicide layer is widely used as a conductive layer so as to improve the electrical conductivity of a word line by combining a high resistance characteristic of the polysilicon layer and a low resistance characteristic of the tungsten silicide layer.
The deposition of the tungsten silicide layer is generally carried out by a CVD (chemical vapor deposition) process, wherein hexafluoride (WF6) gas is deoxidized using monosilane (SiH4) gas, hydrogen (H2) gas or silicon (Si).
According to the conventional method, the density of fluorine atoms accumulated on the tungsten silicide layer exceeds 1.0xc3x971020 atoms/cc when the tungsten silicide layer is deposited on the polysilicon layer by deoxidizing the hexafluoride (WF6) gas using the monosilane (SiH4) gas.
The high-density fluorine atoms accumulated in the tungsten silicide layer causes the diffusion of boron (B) thereby lowering the device characteristics. Particularly, in the case of a gate electrode, the threshold voltage of a transistor is shifted and the thickness of a gate oxide layer increases.
In addition, step coverage and an adhesive feature of the deposited tungsten silicide layer are poor. In order to solve the above problems, a post annealing process is carried out. However, the tungsten silicide layer can be cracked and delaminated while the post annealing process is being carried out.
Recently, dichlorosilane (SiH2Cl2; DCS) gas is widely used as a deoxidizing gas for depositing the tungsten silicide layer so as to solve the problems caused by the monosilane gas.
If the WF6 gas is deoxidized using the DSC gas, the density of the fluorine atoms accumulated in the tungsten silicide layer is reduced by 1.0xc3x97103 times as compared with the density when the monosilane (SiH4) gas is used as the deoxidizing gas. Further, the step coverage and the adhesive characteristic against the polysilicon layer can be improved.
However, though the characteristics of the tungsten silicide layer can be improved, if the tungsten silicide layer is used for the gate electrode having a multi-layer structure of polysilicon/tungsten silicide which requires a re-oxidation process after a patterning process is performed, voids are created on the polysilicon layer while the above processes are being carried out. The voids deteriorate the reliability of the device.
FIG. 1 is a view showing a structure of a gate electrode layer of DRAM device. In order to fabricate the gate electrode layer, a gate oxide film 12, a polysilicon layer 14, a tungsten silicide layer 16, a nitride film 18 and an oxide film 20 are sequentially stacked on a substrate 10 and then the stacked layers are patterned. The polysilicon layer 14 and the tungsten silicide layer 16 are used as a gate conductive layer and the nitride film and the oxide film are used as a mask layer.
After forming the gate electrode layer, a re-oxidation process is carried out. At this time, chlorine atoms contained in the dichlorosilane gas are accumulated on the polysilicon layer 14. When the re-oxidation process is carried out, silicon diffuses from the polysilicon layer 14 into the tungsten silicide layer 16 so that voids 22 are created. The chlorine accumulated on the polysilicon layer facilitates the creation of the voids 22.
In order to reduce the creation of the voids 22, as shown in FIG. 1, a thermal oxidation process was carried out with respect to the monocrystalline silicon wafer 10, so that the oxide film 12 having a thickness of 100 xc3x85 was formed. Then, the doped polysilicon layer 14 having a thickness of 1,000 xc3x85 was formed by means of an LPCVD process. After that, the tungsten silicide layer 16 was formed on the polysilicon layer 14 by introducing a mixing gas of the WF6 gas and the DCS gas. Then, the nitride film 18 and the oxide film having a thickness of 3,000 xc3x85 were formed so as to be used as the mask layer.
After that, the gate electrode layer was defined by coating a photoresist film on the resultant structure, exposing the photoresist film and then developing the exposed photoresist film. The gate electrode layer was patterned by etching a lower layers of the resultant structure through a dry etching process.
Then, as shown in FIG. 2, the oxidation process was carried out for 60 to 100 minutes in an O2 atmosphere at a temperature of 850xc2x0 C. As a result, a thermal oxide film 24 was formed at a sidewall of the gate electrode layer.
Then, as shown in FIG. 3, the nitride layer 18 and the oxide layer 20 as mask layers were etched using a wet etching process. After that, as shown in FIG. 4, the tungsten silicide layer 16 was selectively etched.
When the surface of the exposed polysilicon layer 14 was observed using analyzing apparatus such as SEM/TEM/AFM, the voids 22 were detected.
As mentioned above, when the tungsten silicide layer is deposited according to the conventional method, a great amount of dichlorosilane is introduced into the process chamber at an early stage so that chlorine atoms are accumulated on the polysilicon layer 14. Therefore, when the re-oxidation process is carried out, silicon diffuses through the tungsten silicide layer so that voids 22 are created. The chlorine atoms accumulated on the polysilicon layer 14 facilitate the creation of the voids 22.
The voids 22 created on the polysilicon layer 14 deteriorate the reliability of the device and cause the failure of the device thereby lowering the yield of the device.
The present invention has been made to solve the above problems of the prior art. It is an object of the present invention to provide a method for depositing a tungsten suicide layer. The tungsten silicide layer is deposited by deoxidizing a WF6 gas using a DSC gas on a polysilicon layer. The surface of the polysilicon layer is pre-treated using a hydrogen compound gas including any one selected from the group consisting of group III elements and group V elements of the periodic table of the elements before the tungsten silicide layer is deposited, thereby preventing voids from being created at the polysilicon layer.
In accordance with the invention, there is provided a method for depositing a tungsten silicide layer on a substrate coated with a polysilicon layer in a CVD process chamber. A surface of the polysilicon layer is pre-treated by introducing into the CVD process chamber a hydrogen compound gas including elements selected from group III elements or group V elements of the periodic table. Then, the tungsten silicide layer is deposited on the polysilicon layer by introducing a silane source gas and a tungsten source gas into the CVD process chamber.
For example, the hydrogen compound gas may be PH3 (phosphine), B2H6 (diborane), AsH3, or a mixture thereof. A hydrogen compound gas made of elements selected form group V elements is used for NMOS and a hydrogen compound gas made of elements selected form group III elements is used for PMOS.
As a silane source gas, a dichlorosilane gas (SiH2Cl2) may be used. As a tungsten source gas, a tungsten hexafluoride (WF6) gas may be used.
Since the hydrogen compound gas including elements selected from group III elements or group V elements of the periodic table is absorbed on the surface of the polysilicon layer and the DCS gas is reacted with the absorbed hydrogen compound at the surface of the polysilicon layer, fluorine atoms are not accumulated on the polysilicon layer.
An inert gas such as a He gas or an Ar gas may be mixed with the hydrogen compound gas when the pre-treatment step is carried out.
In one embodiment, the hydrogen compound gas is introduced into the CVD process chamber for 1 to 120 seconds. A flow rate of the hydrogen compound gas introduced into the CVD process chamber is in a range of 1 to 500 sccm. A temperature of the substrate is maintained in a range of 100 to 700xc2x0 C. when the hydrogen compound gas is introduced into the CVD process chamber. Preferably, the hydrogen compound gas is introduced into the CVD process chamber more than 10 seconds, the flow rate of the hydrogen compound gas introduced into the CVD process chamber is in a range of 20 to 200 sccm, and the temperature of the substrate is maintained above 400xc2x0 C. when the hydrogen compound gas is introduced into the CVD process chamber.
When a nucleus of the tungsten silicide is formed, the dichlorosilane (DCS) gas and the WF6 gas are introduced into the CVD process chamber in a flow rate ratio of about 100:1 to 500:1, preferably 100:1 to 300:1. At this time, the chlorine radicals dissociated from the DCS gas are reacted with hydrogen radicals absorbed on the surface of the polysilicon layer. Accordingly, the chlorine radicals are transformed into HCl so that the chlorine atoms are removed without permeating into the polysilicon layer.
In one embodiment, an inert gas such as an argon gas or a nitrogen gas is used as a carrier gas. A PH3 gas can be added to modify a grain size of the deposited tungsten silicide. Therefore, the composition ratio of W/Si can be adjusted so that a thin film having a uniform composition ratio of W/Si in the thickness direction (or the deposition direction) of the deposited thin film can be fabricated. In this manner, after the nuclei of the tungsten silicide have been formed, the variation of the thickness of the polysilicon layer, that is, the reduction of the thickness of the polysilicon layer, can be prevented when the tungsten suicide layer is deposited on the polysilicon layer.
While the deposition step is being carried out, the temperature of the substrate is maintained in the range of about 500 to 700xc2x0 C., preferably, about 570 to 650xc2x0 C. The pressure in the CVD process chamber is maintained at the range of about 0.5 to 10 Torr, preferably about 0.7 to 9.5 Torr.